When an oil or gas well is being serviced a device referred to in the trade as a "tree saver" or wellhead isolation tool is often used. These tools utilize a packoff nipple of a size to fit the tubing or well casing. Typically, the nipple is cylindrical in shape as is the well casing that they seal in. The outer surface of the nipple has a rubber sleeve bonded to a steel surface of the nipple with the outside diameter of the rubber sleeve being somewhat larger than the inside diameter of the well casing in which the nipple is installed so that when the nipple is first inserted by the tool, a slight seal is effected. Subsequently, when fluid or gas is pumped through the tool at high pressure, this pressure is applied outwardly against the skirt of the rubber sleeve to force it against the wall of the well casing. An improved type of nipple is shown in my Canadian Pat. No. 1,169,766 issued June 26, 1984. The nipple of the above-mentioned Canadian patent is inserted by a wellhead isolation tool with an insertion drive system of the type illustrated in my copending Canadian application Ser. No. 444,889 filed Jan. 9, 1984. As mentioned above, the packoff nipples are made slightly oversize in relation to the inside diameter of the well casing in which they are to seal. The packoff nipple is then forced through the wellhead valves and fittings and landed in place in the casing. While wellhead valves and fittings have tightly controlled inside diameters, wellhead casings do not. Accordingly it can happen that a packoff nipple that will fit through the wellhead assembly will be of insufficient diameter to seal tightly against the inner wall of the well casing after it has landed. Conversely, some packoff nipples will fit very tightly into the well casing and may even be damaged by being forced in. This is due to the fact that in many instances, well casing is not perfectly round or it may be corroded or washed due to well fluid properties and characteristics and it therefore becomes difficult to obtain a good seal with a standard size nipple.
The present invention provides a concentric mandrel assembly for use with the wellhead isolation tool and includes an outer mandrel of the assembly carrying a packoff nipple, including a resilient seal member, on its lower end and an inner mandrel of the assembly carrying an expander on its lower end. Means are provided for axially moving the mandrels relative to one another so that the expander can apply outward force on the resilient seal against the inner wall of a well casing. The mandrels can be locked in relation to each other to hold the nipple in the expanded position and the nipple can be subsequently released when it is required to pull it out of the well casing. The mandrel assembly is actuated from outside the wellhead and well casing, and independent of the conditions inside the well. Due to the use of an expandable member, a much more effective seal can be applied against the inside wall of the casing and a casing that is slightly off round or has surface imperfections, can be accommodated.